Transmissive Liquid Crystal Devices (LCDs) and other types of display devices require a backlight illumination source of some type. There are basically three illumination technologies in contention for the backlighting market: Electroluminescent Lamp (EL), Cold Cathode Fluorescent Lamp (CCFL), and Light Emitting Diode (LED). To date, CCFL technology has enjoyed the bulk of the laptop and portable display market, providing highly efficient and dependable light sources. CCFLs are particularly adaptable to edge-lit applications, in which these linear light sources direct light into one edge of a plate or film that spreads the light over its output surface. However, there are inherent drawbacks to CCFL technology that limit its projected growth. For example, CCFL lamps contain mercury and are somewhat fragile. CCFLs are available and are practical only in a limited range of sizes, constraining their usefulness for very small displays as well as for larger displays, such as those preferred for television viewing. Additionally, CCFLs generate unwanted heat in backlight units, potentially warping or otherwise damaging one or more of the optical film components located in the display module.
LED backlighting has inherent advantages over these other technologies. LEDs are mechanically robust, and require only low DC voltage sources. Suitable types of LEDs can be extremely bright, relatively efficient, and have inherently long life. Available in various colors, LEDs offer advantages of larger color gamut due to narrow spectral characteristics and allow easier manipulation of color. While LEDs can also be deployed in edge-lit apparatus, they also have advantages over other technologies for direct view illumination apparatus, in which an arrangement of light sources spaced apart over a surface provides the needed backlight source.
A number of direct view LED backlighting solutions have been commercialized, including the device used in the LNR460D LCD flat-screen HDTV from Samsung, for example. Patent literature describes a number of LED backlight arrangements and improvements, for example:
U.S. Pat. No. 6,789,921 entitled “Method and Apparatus for Backlighting a Dual Mode Liquid Crystal Display” to Deloy et al. describes an LED backlighting arrangement using multiple two-dimensional LED arrays including heat sink compensation;
U.S. Pat. No. 6,871,982 entitled “High-Density Illumination System” to Holman et al describes a backlight having an array of LEDs positioned within reflective housings and having supporting prismatic films;
U.S. Pat. No. 6,568,822 entitled “Linear Illumination Source” to Boyd et al. describes an illumination source for improved uniformity using LEDs, each partially enveloped within the notched input surface of a lens element;
U.S. Pat. No. 6,666,567 entitled “Methods and Apparatus for a Light Source with a Raised LED Structure” to Feldman et al. describes an LED backlight that provides added luminance by positioning LED devices above a reflective surface and providing supporting optics for spreading light from the LEDs; and,
European Patent Application publication No. EP 1 256 835 entitled “Backlight for a Color LCD” by Paolini et al. describes an LED backlight arrangement wherein light from side-positioned LEDs is redirected outward by structures spaced within a light guide.
While each of the above-listed solutions promises at least some measure of improved backlighting performance using LED sources, there are drawbacks with each of these solutions and there still remains considerable room for improvement. Achieving uniformity over an area from point light sources is a complex problem and requires the interaction of multiple optical components, both for spreading the illumination over a broader area and for directing the light toward the backlit display with suitable directivity. Some combination of optical components would be required to spread and condition the point source LED illumination suitably for a backlit display.
While there has been considerable attention paid to LED backlighting devices, a number of drawbacks remain. Because LEDs act substantially as point light sources, LED direct-view backlights require high-performance diffusive elements to diffuse light over a broad surface area and recycle light where necessary. This adds to the thickness and expense of an LED backlight. Heat from the LEDs themselves can also be a problem. Hot spots from these light sources can cause uniformity aberrations in the LCD. Other illumination non-uniformities result from the overall poor light distribution of many conventional systems.
Thus, it can be seen that there would be advantages to a direct view LED backlighting apparatus that exhibits improved uniformity and efficiency, lower cost, and thinner dimensional profile.